Process of devulcanizing rubber



To all whom it may concern or other compounds,

No Drawing. Application filed October UNITED'YSTATES PATENT OFFICE;

WILLIAMS BEACH PRATT, OF WELLESLEY, MASSACHUSETTS, ASSIGNOR TO BONDOUT RUBBER COMPANY, OF EAST DEIDHAM,

NEW YORK.

MASSACHUSETTS, A CORPORATION OF 23, 1914, Serial No.

Be it known that I, WILLIAM BEACH PRATT, a citizen of the United States, and resident of Wellesle in the county of Norfolk and State of liiassachusetts, have invented certain new and useful Im rovements in Processes of Devulcanizing ubber, of which the following is a specification.

Generally speaking, this invention has for its object the devulcanization or reclaiming of rubber, or, in other words, its purpose is to remove from vulcanized rubber more or less of the sulfur which was previously incorporated in the rubber ini the process of vulcanization. In the practical use of rubber in various arts, it is employed in connection with fabrics, as for instance in the manufacture of tires, rubber boots and shoes; it is compounded with resins or asphalts an other hydrocarbons, and it has mixed with it various elements, such as pigments, etc.

While the invention, broadly considered, has for its purpose the removal of sulfur and the restoration of the vulcanized rubber to a state in which it possesses substantially the initial physical and chemical characteristics of pure rubber, nevertheless another and less important phase of the invention consists also in the elimination or removal of the foreign materials, whether they be organic, or inorganic, such as cellulose'fibers, litharge as 'asphalts or other hydrocarbons.

In the vulcanization of rubber, themineral sulfur is incorporated in the body of the rubber and the mixture is subjected to a temerature which ranges from 120 C. to 160 C. The subjection of the mixture to heat is 7 continued for varying periods, according to the particular character of the compound or mixture, and the use to which the final prodnot is to be put. I have discovered that, by the employment of an organic acid soluble in water, as for-instance an organic dibasic acid, as a reagent, Iam'able to separate or free combined sulfur or sulfur of vulcanization from vulcanized rubber. By use of an acid of this character, sulfur, which was added and combined molecularly with the rubber in the vulcanizing process, may be separated from the rubber molecule and then.

Specification of Letters Patent.

moved, practically all in one treatment.

Patented June 28, 1921.

Serial No. 868,205. Renewed December 1, 1920. 427,656.

left in the free mineral state (or combined with the metals or their oxids which are present in the stock or which may be added to the stock), or else removed subsequently by mechanical means, or essin which is utilized a reagent, such as acetone. If desired, a solvent for free sulfur may be introduced into the acid solution, so that the sulfur as soon as freed may be re- As a result of numerous experiments," I have discovered that desulfurization apparently takesplace under similar conditions of heat and duration of time that thevulcanization was originally erally known just what chemical reaction takes place in the vulcanization of rubber; but it is known, however, that at the temd peratures of vulcanization, the sulfur as-' sumes a colloidal or quasi-colloidal condition, and it is at the tlme when the sulfur is in this condition that the molecular rearrangement takes place. Irrespective of the theory of the chemical changes during the time of vulcanization, I have noted that, for the successful separation of combined sulfur in commercial quantities and at relatively low cost, approximately that temperature may be employed which will bring sulfur to a colloidal or quasi-colloidal state. That is,

by a chemical procobtained. It is not gen- I have discovered that, if vulcanized rubber be heated in the presence of an organic acid soluble in water to approximately the temperature of vulcanization, a chemical reaction takes place, the result of which is the splitting off of sulfur from the rubber molecule, making it possible to recover sulfur in a free mineral state. One of the dangers to be guarded against in the process of devulcanization is the decomposition of the reagent. For instance, if oxalic acid, which decomposes at a temperature of approximately 160 care must be taken that the temperature of C., is employed as the reagent,

devulcanization does not so closely approximate 160 C. as to cause the splittin upor decomposition of the oxalic acid. or all practical purposes, taking the various waste rubber productsas they come, I find that on a commercial basis the devulcanizatio-n may be carried on practicallyand satisfactorily at depend, roughly,

' stance, where a under differing temperatures ranging from 120 C. to 140 en oxalic acid is used, and the same is probably true of other organic acids of the class specified, it may be employed in a hi .hly concentrated state, or, if desired, in a re atively weak solution,and the period of time to accomplish the devulcanization will upon the strength of the solution that is employed and the temperature at which the stock is cooked. For in- 7 weak solution (for example, a saturated solution of oxalic acid and water at a normal temperature, say 20 C.) is employed, the period of time required for devulcanization is approximately 12 hours, at .a temperature of 120 (1, whereas, when the solution is increased in of time required for devulcanization at the temperature mentioned decreases, all other things being equal.

Commercial rubber articles are vulcanized conditions, e. 9., one manufacturer may employ approximately 20% by weight of sulfur in the vulcanization of the rubber, and subject the compound to the vulcanizing temperature for a period of approximately 15 minutes, and another manufa'cturer may, on the other hand, emplo 10% by weight of the sulfur and subject 1t to a vulcanizing temperature for an hour or more. In these two articles, therefore, there is a material difference in the proportion of combined sulfur in the rubber, and, as aplarger quantity of 'Should use quantitatively the reagent necessary to abstract of ree and combined sulfur in the rubber,-

parently in the devulcanization a chemical change takes place, there is a necessity, in carrying on the process on a large commercial basis, for the use of a proportionately the reagent, oxalic acid, than mlght in any one case be required for removal of the combined sulfur from the rubber. oreover, as the sulfur will not again combine with the rubber in the presence-of the dibasic acid in vulcanizing temperatures, there should preferably be always an excess of this reagent. Theoretically, one amount of the the quantity leave a slight so that there plus an amount necessary to excess of acid 1n the solution with the rubber at the temperature to which action upon Of the organic 1 of those cellulose materials which are employed in practical rubber industries. That is to say, by the employment of the same known to me strength, the period water, roughly speaking,

, ber.

- add to the acid solution a small say 2 pounds for 100 pounds of rubber stock,

reagent, I am not only able to devulcanize the rubber but also ab!" to remove the oellulose materials which were previously incorporated therein, in the manufacture of those Waste products which are used in following out my process. The effect of the oxalic acid upon these cellulose fibers, asis well known, is to leave them in a charred or friable condition so that they may be easily washed out mechanically.

s an example of the best method now of carrying out my process commercially, I present the following: Rubber articles to be treated, say worn out automobile tires or the like, are first disintegrated b grinding and are placed in a digester. T0100 pounds of the stock I add 50 pounds of crude oxalic acid, and suflicient water to cover the stock. This amount of produces a substantially saturated solution at 80 C, The tank is then sealed. By a suita'ble heater, the contents of the digester are slowlyheated under pressure to a temperature of approximately Then the stock is removed from the liquor and is subjected to a series of washings to remove the charred or friable cellulose fiber and that free mineral sulfur and compounding material which are adherent to the rub- The free sulfur may also be removed by a suitable solvent. When the rubber is Washed and the inorganic matter has been removed thereby, the rubber innnediately swells and will be found to have a specific gravity less than that of water.

In actual practice, I have at times found it desirable to introduce an agent 'to softenthe rubber. I have found that the terpenes, for instance turpentine or pine oil, may be employed. The stock may be previously treated with a small quantity of turpentine or a soluble pine oil before its treatment with acid: but I have found it convenient to quantity,

of commercial pine oil. As a result, the

rubber is softened and sulfur is freed or separated in visible particles. In this form the sulfur is more easily removed by the subsequent washing.

During the cooking process, the threads and other cellulose fibers, such as the canvas duck which is incorporated in vehicle tires, will be disintegrated andin some cases carbonized, and the foreign substances, such as pigments, resins and hydrocarbons, will se arate fromthe rubber and will sink to the held in suspension bottom of the liquor or be therein, according to their natures.

F ram the waste liquor, I may subsequently? sulfur, for

reclaim the sulfur, litharge, hydrocarbons, etc., but this step forms no part of the present invention.

The commercial stock contains usually more or less litharge, zinc oxid, copper (from rivets), iron and other metals, either as such or as oxids, and it is probable that they further combine with the abstracted I have found, by analysis of the Waste products, all of these elements present in combination .with sulfur. I may add therefore a metal to the stock which will combine with the sulfur to form a sulfid or sulfate.

I have herein described one process in.

which the stock is subjected to heat in the presence of oxalic acid, but it is to be understood that my invention is not limited thereto. In fact, it is possible to practice my invention without usin heat or pressure, but in such case the time c ement is reater, and as a result the product is not 0 tained at the low cost which seems desirable. For instance, I have found that simple immersion in a concentrated aqueous solution of oxalic acid, if continued for a long time, say seven to ten months, will result in the complete devulcanization of the stock. Again I prefer to rind the stock fine, but it is not essential or some urposes that the stock be ground at all. have found that whole vulcanized articles may be treated merely for the purpose of counteracting or compensatin for over-vulcanization and oxidation. ere vulcanized rubber articles are oxidized or are over-vulcanized, 2'. c. are subjected to too great heat or pressure or are compounded with too much sulfur,- immerse them in a bath of aqueous solution of oxalic acid for a short time, say one to three hours according to size, and then wash them in water, afterwhich the rubber will be found to possess the characteristics of properly vulcanized new rubber. Care must be taken in such cases not to continue the immersion too long. I

Heretofore it has been proposed by the use of actetone or other like reagents to abstract from vulcanized rubber the free sulfur, and it has been known that by the use of sulfuric acidcomlqined sulfur has been abstracted from vulcanized rubber. In the one case, however, the combined sulfur is not affected by the reagent, and in the other case the sulfuric acid is apt to injure the rubberas a commercial product, and changes its chemical and physical characteristics.

I believe myselfto be the first to have provided any process of freein or removing from vulcanized rubber sulfur which was combined therewith in the process of vulcanization and of thereby reclaiming the rubber or restoring it to substantially its original chemical and hysical condition. Consequently I do not llrnit myself to the particular steps which I have herein described nor to the exact reagents which I have herein named, since I recognize that equivalents therefor may be employed Without departing from the spirit and scope of my invention. Again, so far as certain phases of my invention or discovery are concerned, it is not essential that the process be carried on so far as to completely remove the combined sulfur, as it Ina be employed merely for counteracting oxi ation or overvulcanization as hereinbefore described, or else for separating only a portion of the comzined sulfur from the rubber mass.

So far as any generic features are concerned, this is a continuation in part of my application Serial No. 765,871, filed May 6, 1913.

I claim: I

1. The herein described process of treating vulcanized rubber, which consists in subjecting such rubber acid.

2. The herein described process of reclaiming vulcanized rubber, which consists in to the action of oxalic ganic acid soluble in water which will react on the combined sulfur.

3. The herein described process of devulcanizing vulcanized rubber, which consists in heating such rubber to a temperature at which sulfur becomes colloidal, splitting of! molecularly combined sulfur by an organic acid soluble in water, and -maintaining such temperature until combined sulfur is molecularly freed from the rubber by said acid.

4. The herein described process of devulcanizing vulcanized rubber, which consists in heating such rubber in a closed digester to the temperature of vulcanization, @in the presence of an organic acid soluble in water capable of acting'on the combined sulfur.

5. The herein described process of devulcanizing vulcanized rubber, which consists in heating such rubber in a closed digester to the tem erature of vulcanization, in the presence 0 an organic dibasic acid capable of acting on the combined sulfur.

6. The herein described process of devulin gradually heating such rubber in a closed digester and in a solution of oxalic acid to a temperature at which rubber vulcanizes, maintaining such temperature for a predeterminedv period, and then washing the rubber.

llt

8. The herein described process of reclaiming rubber from waste vulcanized rubber products containing cellulose, which not less than 120 "excess consists in reducing such products to small particles, immersing the stock in a solution of an organic acid soluble in water, heating the same in a digester to a temperature of until the cellulose is disintegrated or made friable and combined sulfur freed from the rubber, and then washing the stock to clean the rubber.

'9 he herein described process of devujlcanizing vulcanized rubber, which consists in heating vulcanized rubber at a.v'ul-' canizing temperature in the presence of an quantity of an organic acid soluble in water.- I n 10; The herein described process which consists in subjecting vulcanized rubber to the action of an organic acid soluble in water capable offreeing the combined sulfur, in the presence of a solvent capable of softening the rubber.

11. The herein described process which consists-in treating vulcanized rubber with a j solvent, and-subjecting the same to the action of an organic acid soluble in water.

12. The herein' described process which consists in treatingvulcanized rubber with a 1 solvent and with oxalic acid under-heat and pressure.

13.- The herein described process which consists in freeing sulfur from the vulcan- MARCUS B. MAY, P. W; "Pnzzm'rr. 

